In certain video disc systems, video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc record surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectric material. A signal pickup subject to engagement with the spiral groove includes a conductive surface which cooperates with the conductive coating and the dielectric deposit of the disc record, to form a capacitor. When the disc record is rotated, an edge of the conductive surface of the signal pickup, while riding in the disc record groove, recovers capacitive variations due to geometric variations in the bottom of the smooth spiral groove. The capacitive variations, which represent prerecorded video information, are processed and applied to a conventional television receiver for reproduction. The variable capacitor concept is described in detail in the copending U.S. Pat. No. 3,842,194, issued to J.K. Clemens, entitled "INFORMATION RECORDS AND RECORDING/PLAYBACK SYSTEMS THEREFOR", and assigned to the present assignee.
The capacitive variations, between the conductive coating on the disc record and the conductive surface of the signal pickup, are a part of a tuned circuit. The resonant frequency of the tuned circuit varies as the capacitance between the signal pickup and the disc record changes. A flexibly mounted conductive member (e.g., pickup arm) is provided; with its free end electrically and mechanically coupled to the conductive surface of the signal pickup, and its pivoted end movably connected to a hollow box-like support housing also made of conductive material. The conductive member mechanically supports the signal pickup, and also forms a transmission line with the surrounding conductive support housing. The tuned circuit includes the reactance of the transmission line and the capacitance between the signal pickup and the disc record. The tuned circuit is energized by a fixed, high frequency (for example, 915 MHz) oscillator. The response of the tuned circuit to the oscillator excitation signal changes as a function of prerecorded information on the disc record. The output signals from the tuned circuit, indicative of information prerecorded on the disc record, are subsequently applied to a suitable signal processing circuit coupled to a television receiver. The transmission line concept is described in detail in the U.S. Pat. No. 3,872,265, issued to S. E. Hilliker, on Mar. 18, 1975, entitled "VIDEO DISC TRANSMISSION LINE AND STYLUS RF RETURN SYSTEMS", and assigned to the present assignee.
The support housing, carrying the pickup arm assembly, is mounted on a suitable feed drive mechanism for driving the pickup arm assembly transversely across the disc record in proper time relationship with the rotational speed of the disc record. The feed drive mechanism drives the signal pickup radially inward toward the center of rotation of the disc record such that the attitude of the signal pickup conductive surface is held relatively constant with respect to the groove. It must be noted that a positive feed drive mechanism is desirable as relatively fragile walls of video disc record grooves, (for example, in a disc record having 4,000 to 6,000 grooves per inch) used in the aforementioned Clemens' type systems, cannot be dependably relied upon for tracking (that is, pulling the pickup arm assembly across the recorded surface of the disc record). It is an object of the present invention to provide an improved and novel feed drive mechanism which is simple in construction, easy to manufacture at a low cost, reliable in operation, and compatible with the requirements of video disc systems.
In a system using the above-discussed high frequency (for example, 915 MHz) transmission line and associated circuits it is desirable to provide effective shielding for radiation at the frequencies employed in order to avoid deleterious interference with the ambient electronic equipment. An advantageous shielding arrangement can be obtained when the supporting housing and the pickup arm circuits are semi-permanently sealed in a metal enclosure, such as a lid of the playback apparatus with an appropriate opening for allowing the signal pickup to ride in the disc record groove, as disclosed in the copending application of Marvin A. Leedom, Ser. No. 557,180, entitled "OVERHEAD SIGNAL PICKUP DRIVE AND DISC RECORD GROUNDING APPARATUS" and concurrently filed herewith. One of the aspects of the present invention is provision of a novel radial drive mechanism for the support housing carrying the pickup arm assembly which is compatible with the aforesaid advantageous shielding arrangement for a video disc system.
In a conventional playback apparatus the pickup arm carrying the signal pickup moves to and from a rest position which is located at a distance greater than the disc record radius from the center of the turntable. Therefore, each time a disc record is played, the pickup arm makes wasteful movements to clear the disc record played: for example, from the rest position to a play position at the beginning of a playback cycle and from the play position to the rest position at the end of a playback cycle. Another purpose served by the present invention is elimination of these wasteful movements of the pickup arm assembly. The novel drive mechanism of the present invention permits location of the pickup arm assembly in the lid whereby it is unnecessary for the pickup arm to clear the disc record between playback cycles.